Hydraulic control valve



July 31, 1962 J, Q FREEBORN 3,046,951

HYDRAULIC CONTROL VALVE Filed March 27, 1961 INVENTOR.

JOHN C. FREEBORN F1'7.Z BY g I f ATTORNEY iinite States ifiatent 3,046,951 HYDRAULIC CONTRQL VALVE John C. Freeborn, R chfield, Minn assignor to Minuteapolis-Honeywell Regulator Company, Minneapolis, Minn., a corporation of Delaware Filed Mar. 27, 1361, Ser. No. 98,688 6 Claims. (Cl.-121-15( My invention relates to hydraulic control valves of the time modulated type and more particularly to a hydraulic type multivibrator with simplified electric control.

Time modulated hydraulic control valves are known and used, but they generally have limited application because of the complexity of the structure involved. Valves of this type generally employ either complex electrical control circuitry for vibrating the hydraulic control valve or complex hydraulic switching circuitry to provide the wave generation output from the hydraulic valve. While apparatus of this type is desirable from the standpoint of accurately positioning and controlling loads, the complexity of the structure has generally limited the application of this type of control. The present application is directed to an improved type of hydraulic control valve or hydraulic multivibrator which has a square wave type output for time modulation control on associated devices. This application is an improvement over the'copending application of Robert V. Burton, now Patent 2,996,045, dated August 15, 1961, on a Time Modulated Hydraulic Servo Valve. In the Burton application the switching of the main control valve is done through auxiliary switching and timing valves in accordance with the controlled flow through regulated orifices. In'the present invention the switching of the main control valve is done through regulation of pressure applied to the motive section of the control valve and an adjustment of the bias on the pressure responsive switching valves associated therewith. Accurate control of the electrical portion of the network is not required and a small amount of current or signal will provide an accurate control over the operation of the hydraulic multivibrator.

It is therefore an object of this invention to provide an improved hydraulic control valve of the time modulated type or a hydraulic multivibrator type control. It is also an object of this invention to provide a valve of this type in which the switching is controlled from the pressure of 1 the hydraulic fluid with small electric signals controlling the shape of the output wave form of the hydraulic multivi=brator by adjusting bias on a pressure responsive switching valve. These and other objects of this invention will become apparent from the reading of the attached description together with the drawing wherein:

FIGURE 1 is a schematic view of the improved hydraulic multivi'brator, and

FIGURE 2 is a schematic wiring diagram of the electrical portion of the hydraulic multivibrator.

My improved hydraulic multivibrator or time modulated control valve is shown schematically in FIGURE 1 as including a main control valve indicated at 10, this control valve being basically a four-way valve of conventional configuration. Control valve 10 includes a casing 11 and a spool, indicated generally at 12, this spool having four land portions 14, 15, 16 and 17 with dwell portions '18, 19 between the land portions. Casing 11 includes a plurality of conduits connected thereto including a supply pressure conduit indicated at 20, return line conduits indicated at 21 and a pair of cylinder or outlet conduits indicated at 22, 23 respectively. The operation of this control valve is conventional with four-way valve operation, in that movement of the spool 12 will position the lands and 16 relative to the outlet conduits 22, 23 such that one or the other will be connected with supply conduit through the dwell portion 19' while the opposite outlet conduit will be connected through the dwell portion 18 outside of the lands 15, 16 to the return conduit 21. The valve spool 12 is moved within the cylinder casing 11 by means of pressure diflerential applied to the motive means or ram portions of the valve. These motive means or ram portions of the valve are defined as a chamber within the casing 11 located to either side of the external land portions 14, 17 of the valve spool. For purposes of identification these ram portions or motive chambers are identified at 25, 26 respectively. The outlet conduits 22, 23 of the control valve are connected respectively to the opposite inlet extremities of the device to be actuated 30, shown herein as a simple bidirectional hydraulic actuator. The actuator is comprised of a casing 31 having inlet extremities at 32, 33 at the extremities thereof to which the outlet conduits 22, 23 of the. control valve are connected. The actuator also includes a piston 35 slideably positioned within the cylinder and defining motive chambers within the cylinder to which the inlets are attached. Piston 35 has output shafts 38 connected thereto or integral therewith which are guided by end plates or guides 40 integral with ends of the casing 31. .The hydraulic output from the control valve 10, which is the main valve of the hydraulic multivibrator, is of the square wave type. That is, its output pressure variations due to control movement of the mova- 'ble element or spool 12 will cause pressure variations to the device 30 to be actuated to provide an ultimate movement of the piston or output shaft 38 in accordance with the over-all pressure differential applied to the extremities of the casing 31. This movement of the device to be actuated will be time modulated in accordance with the length of time of each half cycle of pressure variation in the output wave form of the hydraulic multivibrator.

The operation of the control valve 12 through the motive sections or ram sections 25, 26 thereof is controlled by a timing valve or variable pressure means also connected to the outlet conduits 23, 22 such as is indicated bythe conduits 41, 42 respectively, which include directional check valves 44, 45. The timing valve actually comprises a cylindrical casing 51 having a floating piston 52 therein, the piston being slideably positioned within the casing to define variable pressure chambers such as is indicated at 53, 54 respectively within the casing to either side of the piston 50. Positioned Within the pressure chambers 53, 54 are centering springs 55, 56 respectively, these springs bearing against the extremities of the casing 50 and the piston 52 for centering purposes. The inlet conduits 41, 42 connect to the casing 51 and hence the chambers 53, 54 through inlet conduits indicated at 57, 58 respectively. Also connected to the pressure chambers or the inlet conduits 57, 58 are additional conduit means 60, 61, which include orifices 62, 63. The conduit means 60, 61 provide a means by which pressure within the chambers 53, 54 may be bled to a common conduit 65 connected to the return line' conduit similar to or the same as conduit 21. The orifices 62, 63 are fixed and control a fixed rate of bleed from the pressure chambers 53, 54 depending upon the pressure therein, and the pressure chambers of the timing valve 50 are basically supplied by pressures in the outlet conduits 22, 23 except during the conditions when the respective conduits 22 or 23 are connected to the return line conduit 21. At such a time the check valves 44 or 45 prevent communication of the chambers 53, 54 with the outlet conduit except as through the restrictions 62, 63 and the common conduit 65. 1

In addition to the conduits 41, 42 and 60, 61 leading to and from the pressure chambers 53, 54 of the timing valve or pressure responsive means controlling the opera. tion of the control valve 10, additional conduit means or conduits'70, 71 are connected to these pressure chambers a 3 and lead therefrom through directional check valves 72, 73 respectively in the conduits to the ram sections or rnotive means26, 25 of the control valve to supply the directional check valves 72, 73, the pressure cham bers 54, 53; are directly connected acrossthe control valve for operation of the same, and the pressures in the pressure chambers 53, 54 are imparted to the pressure chambers or ram sections 26, 25 of the control valve. Also included as a part of this'conduit or connection between the pressure chambers 53, 54 and the ram sections 26, 25 of the control valve are pressure responsive switching valves 80, 81 respectively. Each pressureresponsive switching valve includes a casing 82, 83

housing a piston 84, 85 having an operating shaft 86, 87

i associated therewith, the pistons being biased within the respective cylinders 82, 83 by means of springs 88, 89 respectively. In the pressure responsive switching valve 80, the upper extremity of the casing 82 is connected by means of a conduit 90 tothe conduit 70 above the directional check valve 72 and communicating directly with the ram section ormotive means 26 of control'valve 10. Similarly the lower extremity of the piston is connected by means of a conduit 91 to the outlet conduit 70 below the directional check valve. This places thepiston 84 within the cylinder 82 directly across or between the ram section or motive means 26 of control valve and the pressure in chamber 53 of the timing valve 50. The, pressure responsive switching valve 81 similarly has its piston 85 connected across or between the pressure chamber 54 and the ram section or motive means 25 of control valve'10' through a conduit 94 connected to the motivemeans orram section 25 of' the control valve 10 above the directionalcheck valve 73 and the lower eX- tremity of the casing 83 or the piston 85 connected through a conduit 95 to the conduit 71 below the directional check valve 73 and common with the pressure chamber 54 of timingvalve 50. In addition, each switching valve has its respective conduit 70, 71 connected into the side of the cylinder 82, 83 respectively such that the pistonwhen displaced withinthe cylinder will operate as a valve to effect a bypass around the directional check valves 72, 73. The casings 82, 83 with pistons 84, 85 therein provide relatively large volumes for the fluid in the lines to flow into causing a decrease in line pressure whenever the respective pistons are moved suddenly from one extremity of the casing to the other. This will set up a switching action in a manner later to be described. The movement of the pistons will act as the valve to bypass thedirectional check valve and allow a rapid change inpressure in the motive chambers 25 or 26 at the extremities of the control valve 10 under certain conof the control valves or pressure responsive switching valves 80, 31 by applying additional bias to the respective switchingvalves. Each of the electromagnets are controlled respectively by coils' indicate d at 104, 105, the coils being adapted to'be selectively energized with a di; rect current to provide a given attraction between'the a in a closed servo loop, it will be moved to a position in proportion to the differential between on and olf time electromagnet and the pivoted armatures 102, 103 to apply a given force to the respective output shafts or thrust members 86, $7 of the switching valves in a direction to aid the effects of the springs 88, 89.

The schematic wiring diagram in FIGURE 2'shows one way of energizing these coils, this circuit including a transformer 110 having a primary winding 105 adapted to be connected to an alternating current source of power and a secondary winding 106 connected across a full wave rectifier indicated generally at 107. The output diagonals of the full wave rectifierare connected respectively to the extremities of the coils 104, 105 such that an extremity at each of the coils 104, 105 are-connected in common to one output diagonal of the rectifier while the opposite output diagonal is connected through: an adjustable potentiometer indicated at 109 to the opposite extremities of the coils 104, 105 such that the coils may be selectively and variably energized with a direct current source of power in a ditferential manner.

have shown the load 30 herein as a floating piston type actuator 30 which is controlled by valve 10 through the variation in oscillation of the carrier frequency at the output conduits 22, 23. The carrier frequency will control flow to the opposite extremities of the piston within the load device or actuator 30 to v'ary'the rate of movement of the piston in one direction or the other. When the device to be actuated is loaded or connected of the control valve or the relative periods of time inwhich the spool of the control valve is in one position or the other, connecting the pressure supply conduit 20 to the one or the other of the outlet conduits 22; 23'. Thus, the control of the load will be in terms of the pressure differential output existing between the conduits 22, 23 on a time basis. It should be recognized that in hydraulic multivibrators or time modulated hydraulic valves of this type the applied pressure ranges on the system would be between 'ab0ut'700 to 3000'p.s.i. to pro vide an oscillation from the control valve or servo; valve in the range of 200 to 300 c.p.s. The actual'length of' time that the control valve is inIone switching position, or the other, that is applying pressure directly to one output conduit while the other is connected to the return line within a given cycle will determine the actual pressure differential existing in the device to be actuated andhence the ultimate position of the same in a conen-tional time modulation type ofoperation.

7 Thus, in the present device, supply pressure from inlet, supply port 20 is directed through the control valve10 54 of the timing valve 50. Thus in the position or condition of the apparatus shown in FIGURE 1', the pressure chamber 54 of valve is connected to the supply conduit 20 through the outlet conduit 22"anddirectional check valve 45 while the pressure in chamber 53 bleeds down from the pressure level which exists therein'prior to switching. The directional check valve 4 svpreyents1 communication of this chamber with the return line con-,7

duit 21 through the conduit 41. Both pressure chambers 53, 54 are, however, connectedthrough restrictions 62; 63'" to the return line conduit leading to the conduit,21

or the equivalent thereof. Thus, supply pressure will be bled from the chamber 54 through the restriction 63 at the same time that the pressure existing in chamber 53" is bled through the restriction 62 t0 the return lin n. This Movement of the spool or valv'eelef V The outlet conduits 22; 23 are connected in addition to the pressure chambers 53,

timing device will control the pressure differential across the ram sections 25, 26 to provide the square wave alternating pressure output from the control valve. The alternating half cycles of pressure are controlled with respect to time as will be later noted through the energization or operation of the electromagnets 1%, 191. Thus with the control valve in the position shown the hydraulic fluid existing in pressure chamber 53 is forced through the orifice 62 and the piston 52 is moved from left to right compressing the centering spring 55. The pressure in the chamber 53 will decrease and the piston will slow down as the compression of the spring 55 builds up. The pressure in area 54 of the timing valve 50 is communicated or transmitted through the check valve 73 to the ram section 25 and hence through the control valve by virtue of application to the spool 12 to the top of the piston 84 of pressure responsive switching valve 84 through the conduits 70 and 90. The directional check valve 72 prevents this pressure from being transmitted into the pressure chamber 53. Similarly, the pressure of chamber 53 is transmitted through the conduits 57, 70 and 91 to the bottom of the switching piston 84 of switching valve 80. Thus, the pressure differential existing on the timing valve is also applied to the pressure responsive switching valve 86 causing the piston therein to gradually move against the bias of the spring 88 and the bias of the electromagnet 100. When suificient pressure difierential exists between the areas or the pressure chambers 53, 54 and hence the top and bottom of the piston 84 of the switching valve 89, the magnetic attraction of the electromagnet 109 will be overcome and the armature 102 will be forced down away from the electromagnet through the thrust of the shaft 86 attached to the piston and will rapidly move when the magnetic attraction is broken providing a bypass around the directional check valve 72. The bypass connection allows conduit 90 to communicate directly with the conduit 76 and 57'." This action will permit a rapid change in pressure in the chamber 26 or an increase in the pressure difierential existing between the ram sections 25, 26 causing the control valve 10 or the movable element or spool 12 thereof to transfer completely to the right extremity of the casing 11. In this position the supply pressure port 20 will be connected to the outlet conduit 23 and the outlet conduit 22 will be connected through the dwell portion 18 to the return line conduit 21. At the same time the device to be actuated or floating piston actuator 30 will have its respective inlet connections reversed with respect to supply and return lines causing the piston to be subjected to a pressure differential in the opposite direction.

Once transfer of the control valve 10 has been completed pressure differential will no longer exist across the piston 83 of switching valve 89 and it will return to its normal position by means of the light spring 88 while the operation or energization of electromagnet 1% will cause its respective armature or flapper 102 to be attracted thereto. The pressure in chamber 54 may still exceed that existing in pressure chamber 53 by virtue of the compressed spring 55 in chamber 53 and the relatively small volume of oil necessary to move the control valve 10. However, the pressure chamber 53 is now at the supply pressure and pressure chamber 54 sees supply pressure plus a light spring force with this chamber being bled to the orifice 63 to the return line and being sealed directly from the return line through the control valve by the check valve 45. Pressure in the chamber 54 diminishes or is reduced by virtue of the leak through the orifice 63 as the timing valve or the piston 55 thereto moves from right to left just as the pressure in chamber 53 previously was diminished. This diminishing pressure applied to the base or lower extremity of piston 85 of the pressure responsive switching valve 81 will cause the pressure differential to increase between top and bottom of the piston communicating with the ram section 26 and the pressure chamber 54 respectively to overcome the biasing force of electromagnet 101 and to repeat the switching action. Thus, whenever this pressure differential reaches a prescribed value the attractive force of the electromagnet 101 and the spring 87 will be overcome causing the piston to snap down and allowing a bypass around the directional check valve 73 to unload the ram section 25. This will elfect a transfer of the control valve or the spool 12 thereof from right to left reconnecting or reversing the connections between the supply pressure conduit 20 and the outlet pressure conduit 22. Hence the pressure differential to the device to be actuated 30 will be reversed with the next half cycle of output frequency.

The relative strength of the solenoids or electromagnets 190, 101 as controlled by the current flow through the respective coils 104, 105 thereof will be converted into a time modulated hydraulic control adjusting the length of time which the control valve will remain in one switching position or the other and hence the length of each half cycle of frequency of output wave form. This wave form will determine the ultimate pressure differential to be applied through the outlet conduits 22, 23 to the device to be actuated. FIGURE 2 shows merely one means for deriving a direct current of varying amplitude to the respective coils 104, 165 from an alternating current source, it being understood thatthe potentiometer may be adjusted to either side of a normal or central position to selectively vary the magnitude of current flow difierentially through the respective coils to adjust the on or oif time and hence the output wave form of the carrier frequency from this control valve. This time modulated control can be effective without the need for precision hydraulic parts as presently required in devices of this type.

In considering this invention it should be remembered that the present disclosure is intended to be illustrative only and the scope of the invention should be determined only by the appended claims.

I claim as my invention:

1. A hydraulic multivibr-ator comprising: a control valve having a movable element and including a supply pressure conduit, a return line conduit, and a pair of outlet conduits; said outlet conduits being adapted to be connected to a device to be actuated by said hydraulic multivibrator; motive means included at the extremities of the movable element for said control valve and adapted to selectively control the position of the same relative to said conduits to effect switching of a said supply pressure conduit to one of said outlet conduits and said other of said outlet conduits to said return line conduit; pressure actuated timing means including a cylinder having a floating piston therein defining pressure chambers at either extremity of said cylinder and including centering springs in said pressure chambers acting on said piston; conduit means connecting said pressure chambers of said timing means to said outlet conduits; directional check valve means included in said last named conduit means; additional passage means including restrictions connecting the pressure chambers of said timing means to said return line conduit; additional conduit means connecting the pressure chambers of said timing means and motive means of said control valve such that the pressure differential existing in the pressure chambers of said timing means will cause operation 'of said control valve; directional check valve means included in said additional conduit means and preventing flow reversal from said motive means of said control valve to the pressure chambers of said timing means and the restricted passages connected thereto; pressure responsive valve means connected to and normally communicating with the m0- tive means of said control valve and the pressure chamber of said timing means in a bypass connection around said last named directional check valve means, said pres sure responsive valve means being operative upon apredetermined pressure differential to bypass said directional check valve means allowing rapid change in pressure in one or the other of said motive means of said control valve to cause the movable element of said control valve to rapidly move between opera-ting positions and effect switching of said supply pressure conduit to the opposite outlet conduit thereof; and electromagnetic biasing means for each of said pressure responsive valve means including a coil adapted to be selectively energized to determine the pressure diiferential controlling the operation of said pressure responsive valve means and the operation of said control valve.

2.. A hydraulic multivibrator comprising: a control valve having a movable element and including a supply pressure conduit; a return line conduit, and a pair of outlet conduits; said outlet conduits being adapted to be connected to a device to be, actuated by said hydraulic multivibrator; motive means included. at the extremities of the movableelement for said control valve andadapted to selectively control the position of the same relative tosaid conduits to efiect switchingof a said supply pressure conduit to one of said outlet conduits and said other of said outlet conduits to said return line conduit; pressure actuated timing means including means defining variable pressure chambers; conduit means connecting said pressure chambers of said timing means to said outlet conduits; directional check valve means included in said last named conduit means; additional passage means instricted passages connected thereto; pressure responsive v valve mean connected to and normally communicating with the motive means of said control valve and the pressure chamber of said timing means in a bypass connection around said last named directional check valve means, said pressure responsive valve means being operative upon a predetermined pressure difierential to bypass said directionalcheck valve means allowing rapid change in pressure in said pressure chambersof said timing means and one or the other of said motive means of said control valve to cause the movable element of said control valve to, rapidly move between operating positions and, effect switchingof-said supply pressure conduit to the opposite 7 outlet conduit thereof; and electromagnetic biasing means for iea'chtofsaid pressure responsive valve means including a coil adapted to be selectively energized to determine the pressure diiferential controlling the operation of said pressure responsive valve means and the operation of said control valve. 7

3. A' hydraulic multivibrator comprising: a four way control valve having a supply pressure conduit, a return line conduit, and a pair of outlet conduits connected thereto; said outlet conduits being adapted to be connected to a device to be actuated by said hydraulic multivibrator; ,motive means included with said control valve and adapted toselectively control the operation of said valve to effect switching of said supply pressure and said return line conduits between said outlet conduits; pressure actuated timing means including a cylinder having a floating piston therein definingpressure chambers at either extremity of said cylinder; conduit means connecting said pressure chambers of said timing. means to said outlet conduits; directional check valve means included in said last named conduit means; additional passage means including restrictions connecting the pressure chambers of said timing means to said return line conduit; additional conduit;

means connecting the pressure chambers of said timing means and motive means of said control valve such that the pressure differential existing in the pressure chambers of saiditiming means will cause operation of said control valve; directional check valve: means included in said additional conduit means andtpreventing flow reversal from.

said motive means of said control valve to the pressure chambers of said timing means and'restricted passages con nected thereto; pressure: responsive valve means connected" to and normally communicating with the motive means:

selectively energized to determine the pressure differential controlling the operation of said'pressure responsive valve means and the operation of said control-valve.

4. A hydraulic multivibra-tor comprising: a four way control valve having a supply pressure conduit, a return line conduit, and a pair of outlet conduits connected thereto; said outlet conduits being adapted 'to be con nected to a device to be actuated by said hydraulic multivibrator; motive means included with said control valve and adapted to selectively control the operation of said valve to efiect switching of a said supply pressure and said return line conduits between said outletconduits; pressure actuated timing means including means defining variable pressure chambers; conduit means connecting said pressure chambers of said timing means to said outlet conduits; directional check valve means included in said last named conduit means; additional passage means including restrictions connecting the pressure chambers of said timing means to said return line conmeans and preventing flow reversal from said motive;

means of said control valve to the pressure chambers of said timing means and the restricted passages connected thereto; pressure responsive valve means connected to and normally communicating with the motive means of said control valve andthe pressure chamber of said timing means in a bypass connection around said last named directional check valve means; said pressure re sponsive valve means being operative upon a predetermined pressure diiferential to bypass said directional check valve means allowing rapid change'in pressure in said pressure chambers of said timing means and one or the other of said motive means of said control valve to cause rapid operation thereof to eifect rapid'switching of said supply pressure conduit to the opposite outlet conduit thereof; and electromagnetic biasing means for each of said pressure responsive valve means including a coil adapted to be selectively energized to determine the pressure differential controlling the operation of said pressure responsive valve means and the operation of' said control valve.

5. A hydraulic multivibrator comprising: a control valve including a supply pressure conduit, a return line conduit, and a pair of outlet conduits; said outlet conduits being'adapted to be connected to a device tobe actuated by said hydraulic multivibrator; motive means included in said control valve and adapted to selectively control the operating condition of the valve relative to said conduits to efiect switching of a said supply pressure Y and said return line conduits between said outlet conduits; pressure actuated means having a pair of variable pressure chambers connected to said outlet conduits and though restrictions to said return line conduit; additional conduit means connecting said pressure chambers to motive means for said control valve such that the pressure difierential existing in the pressure chambers will cause operation of said control valve; directional check valve means included in said additional conduit means to prevent flow reversal from said motive means of said control valve to pressure chambers; pressure responsive valve means connected to and normally communicating with the motive means of said control valve and said pressure cham ers in a bypass connection around said directional check valve means, said pressure responsive valve means being operative upon a predetermined pressure difierential to bypass said directional check valve means allowing rapid change in pressure in one or the other of said motive means of said control valve to cause rapid operation of said valve and switching of said supply pressure conduit between opposite outlet conduits; and electromagnetic means for said pressure responsive valve means operative when variably energized to vary the pressure difierential at which said pressure responsive valve means responds to control the operation of said control valve.

6. A hydraulic multivibrator comprising: a control valve having a movable valve spool and including a supply pressure conduit, a return line conduit, and a pair of outlet conduits; said outlet conduits being adapted to be connected to a device to be actuated; a ram section included at each of the extremities of the movable spool of said control valve and adapted to selectively control the position of the same relative to said conduits to effect switching of a said supply pressure conduit to one of said outlet conduits and said other of said outlet conduits to said return line conduit; pressure actuated timing means including a cylinder having a floating piston therein defining variable pressure chambers between either extremity of said cylinder and said piston; conduit means connecting said pressure chambers of said timing 1% means to said outlet conduits to provide varying pressures thereto; directional check valve means included in said last named conduit means; additional passage means including ornice means connecting the pressure chambers of said timing means to said return line conduit to bleed pressure from said pressure chambers; further conduit means connecting respectively the pressure chambers of said timing means and ram sections of said control valve such that the pressure differential existing in the pressure chambers of said timing means will cause operation of said control valve; directional check valve means included in each of said further conduit means preventing flow reversal from said ram sections of said control valve to the pressure chambers of said timing means; pressure responsive valve means connected to and normally communicating with the ram sections of said control valve and the pressure chambers of said timing means in a bypass connection around said last named directional check valve means, said pressure responsive valve means being operative upon a predetermined pressure differential to bypass said directional check valve means allowing rapid change in pressure in one or the other of said ram sections of said control valve to cause the movable valve spool to rapidly move between operating positions and effect switching of said supply pressure conduit between the outlet pressure conduits; and electromagnetic biasing means for each of said pressure responsive valve means including a coil adapted to be selectively energized to determine the pressure difterential necessary to operate said pressure responsive valve means to control operation of said control valve.

References Eited in the file of this patent UNITED STATES PATENTS 1,952,690 Strom Mar. 27, 1934 2,223,792 Muir Dec. 3, 1940 2,689,548 Holm et al Sept. 21, 1954 

